This disclosure relates to optical communication systems.
The multiplexing of multiple orthogonal orbital-angular-momentum (OAM) beams has emerged as a possible approach for increasing system capacity and spectral efficiency (SE) in free-space optical (FSO) links (see G. Gibson et. al., OE 12, 5448 (2004) and J. Wang et. al, NP 6, 488, (2012)). Light beams carrying OAM have a variable phase wavefront of the form of exp(i l φ) (l=0, ±1, ±2, . . . ), in which φ refers to the azimuth angle and l determines the OAM value (see L. Allen et. al, PRA 45, 8185 (1992)). One major advantage of OAM is that the OAM modes with helical phase front and “doughnut” intensity shape are intrinsically orthogonal to one another, which indicates that in principle, efficient multiplexing and demultiplexing of multiple independent and spatially overlapping data-carrying OAM beams can be performed with negligible crosstalk (see L. Allen et. al, PRA 45, 8185 (1992) and A. M. Yao et. al., AOP 3, 161 (2011)).
Several reports have shown that OAM multiplexing can be used in free-space links to enhance data capacity and spectral efficiency, including a 2.56 Tbit/s data transmission link with an SE of 95.7 bit/s/Hz by multiplexing 32 OAM modes (see J. Wang et. al, NP 6, 488, (2012)). However, a critical limitation of transmitting OAM-based data channels is the significant performance degradation induced by atmospheric turbulence (see J. Anguita et. al, AO 47, 2414 (2008); G. Tyler et. al, OL 34, 142 (2009); and Y. Ren et. al, CLEO'13, CM2G.4 (2013)). As the orthogonality of multiple co-propagating OAM beams depends on their distinct helical phase-fronts, the refractive index inhomogeneities of the atmosphere will distort these phase-fronts, causing power spreading of each transmitted OAM mode onto neighboring modes, which will essentially result in inter-modal crosstalk between different OAM data channels. These turbulence-induced effects can severely limit the distance and number of OAM beams that can be accommodated in FSO links (see Y. Ren et. al, CLEO'13, CM2G.4 (2013); B. Rodenburg et. al, OL 37, 3735 (2012); and N. Chandrasekaran et. al, Proc. SPIE 8518, 851808 (2012)).
An important goal for the future use of OAM multiplexing in high-capacity FSO links is the ability to compensate the effects of atmospheric turbulence on the data channels, both in terms of degradation of a single data-carrying OAM beam as well as in the crosstalk increase induced by turbulence (see N. Chandrasekaran et. al, Proc. SPIE 8518, 851808 (2012)).